An electrolytically chromated steel sheet is generally required to be excellent in such properties as surface color tone, paintability, printability and corrosion resistance. The surface film on an electrolytically chromated steel sheet comprises two layers: a metallic chromium layer and a hydrated chromium oxide layer formed on said metallic chromium layer. Of these layers, said upper hydrated chromium oxide layer exerts a very important effect on said properties of an electrolytically chromated steel sheet.
More specifically, an amount of the hydrated chromium oxide layer on the surface of an electrolytically chromated steel sheet (an amount expressed by the chromium content in the hydrated chromium oxide layer) exceeding 0.3mg/dm.sup.2 seriously degrades the surface color tone of the electrolytically chromated steel sheet, and causes production of a surface stain. If the amount of a hydrated chromium oxide layer exceeds 0.4 mg/dm.sup.2, paintability and printability worsen considerably. When the amount of a hydrated chromium oxide layer is under about 0.1mg/dm.sup.2, on the other hand, satisfactory corrosion resistance cannot be obtained.
There are available the following two conventional processes for manufacturing an electrolytically chromated steel sheet:
A. A process, known as the dual-electrolyte process, comprising subjecting a steel sheet to a cathodic electrolytic treatment in an electrolyte containing hexavalent chromium at a relatively high concentration, to form a metallic chromium layer only on the surface of said steel sheet, and then, subjecting said steel sheet with said metallic chromium layer formed thereon to a chemical treatment or a cathodic electrolytic treatment in another electrolyte containing hexavalent chromium at a relatively low concentration, to form a hydrated chromium oxide layer on said metallic chromium layer; and
B. A process, known as the single electrolyte process, comprising subjecting a steel sheet to a cathodic electrolytic treatment in an electrolyte containing hexavalent chromium at a relatively low concentration, to form simultaneously a lower metallic chromium layer and an upper hydrated chromium oxide layer on the surface of said steel sheet.
In said process (b) mentioned above, the amount of the hydrated chromium oxide layer exceeds 0.4mg/dm.sup.2, varying with the conditions of cathodic electrolytic treatment, which results in a serious degradation of said properties of an electrolytically chromated steel sheet. It is therefore necessary to adjust the amount of deposited hydrated chromium oxides within a desired range.
The following processes have conventionally been proposed with a view to adjusting the amount of deposited hydrated chromium oxides in an electrolytic chromate treatment of a steel sheet:
1. Process which comprises raising the temperature of an electrolytic chromating bath; PA1 2. Process which comprises adjusting the chemical composition of an electrolytic chromating bath; PA1 3. Process which comprises scraping off part of the hydrated chromium oxide layer on the surface of an electrolytically chromated steel sheet with rolls (refer to the Japanese Patent Publication No.16,334/74); and PA1 4. Process which comprises dipping an electrolytically chromated steel sheet in an electrolytic chromating bath or a chromic acid solution, to dissolve part of the hydrated chromium oxide layer on the surface of said steel sheet. PA1 subjecting a steel sheet to a cathodic electrolytic chromate treatment at a relatively high speed of 400 to 1,000m per minute in a conventional electrolytic chromating bath, to form simultaneously a lower layer of metallic chromium and an upper layer of hydrated chromium oxides on the surface of said steel sheet; and then, subjecting said electrolytically chromated steel sheet to an anodic electrolytic treatment under the following conditions in said electrolytic chromating bath, to adjust the amount of said hydrated chromium oxide layer within the range from 0.1 to 0.3mg/dm.sup.2 : PA1 Bath temperature : room temp. -- 70.degree. C; PA1 Anodic current density : 0.5 - 8A/dm.sup.2, preferably 0.5 - 4A/dm.sup.2 ; PA1 Treating time : 0.5 - 2 sec; PA1 Amount of electricity : 1 - 8 Coulomb/dm.sup.2.
In said process (1) mentioned above, the hydrated chromium oxide layer of an electrolytically chromated steel sheet becomes thin, which gives a better surface color tone and a higher paintability and printability but lowers the electrolytic deposition efficiency of metallic chromium. In order to obtain a metallic chromium layer of a desired thickness, therefore, it is necessary to provide a longer electrolytic chromating time, and hence, the productivity of electrolytically chromated steel sheets is reduced.
In said process (2) mentioned above compounds containing sulfuric acid radicals, silicofluoride and borofluoride are added in relatively large quantities into an electrolytic chromating bath. These additives affect the hardness and the cracking frequency of the metallic chromium layer on an electrolytically chromated steel sheet and the cracking frequency of the hydrated chromium oxide layer formed thereon. Furthermore, fluorine and sulfur, being adsorbed into the hydrated chromium oxide layer, degrade the corrosion resistance of the electrolytically chromated steel sheet. Moreover, the concentration control of these additives is very complicated and is hardly practicable. In an electrolytic chromating bath with the above-mentioned additives of which the concentration has been adjusted to achieve a thinner hydrated chromium oxide layer, the electrolytic deposition efficiency of the metallic chromium layer is not always high, thus leading to a decreased productivity of electrolytically chromated steel sheets.
In process (3) mentioned above, scratches tend to be easily produced on the surface of an electrolytically chromated steel sheet. In process (4) mentioned above, which is rather practical, the slow dissolution of the hydrated chromium oxide layer by dipping results in the necessity of a longer treatment time, and hence of a longer production line.
In view of these facts, there has been wanted a process for manufacturing an electrolytically chromated steel sheet, which comprises simultaneously forming a lower layer of metallic chromium and an upper layer of hydrated chromium oxides on the surface of a steel sheet in a single electrolytic chromating bath, and which permits adjustment of the thickness of said hydrated chromium oxide layer and gives a high productivity, but no such process has yet been proposed.